While the older information below may still help some people, the solution seems to be either log on, scroll to the bottom of the page and find "Delete all forum cookies", this will log you out but should allow you to log in normally after that, as alternative with the forum front page open:

Sorry we are still working on the problem, it just did it to me, one thing that seems to work if your browser allows it is to open the forum in a private or incognito page,

• Firefox or Opera right click on any link and select Private Page,• Chrome Opera right click on any link and select Incognito Page,• for most recent versions of MS Explorer, click on the cog, select safety then Inprivate Browsing, • for MS Edge, click on the three dots right hand side and select Inprivate Browsing

Thanks to Tina for this temporary solution. NB by its nature these pages will not save log in details.